FSH stimulated the synthesis of prostaglandins (PG) E and F2 alpha by rat ovarian granulosa cells. This dose-related effect was augmented by exogenous hCG. Injections of hCG and/or PRL had no effect without simultaneous FSH treatment. However, infusion of PRL into FSH- or FSH/hCG-treated animals exerted marked effects on PG synthesis. Physiologic replacement doses of PRL resulted in a 2- to 3-fold increase in the rates of PGE and PGF2alpha synthesis whereas injections of supraphysiolic amounts of PRL decreased the levels. The effectiveness of PRL over such a narrow concentration range is in agreement with in vitro studies on ovarian steroidogenesis. The role of PG in ovarian function is being studied via a newly devised assay for prostaglandin receptors. The observations suggest that prolactin exerts its effects through the PG cascade and explains, in part, the mechanism by which infertility occurs in hyperprolactinemic women.